1) Field of the Invention
The present invention relates to an optical terminal apparatus that amplifies a signal light by using Raman amplification or an erbium-doped optical fiber amplifier (EDFA).
2) Description of the Related Art
In an optical transmission system using an optical fiber, a multiplexing technique for a signal light by using a wavelength division multiplexing (WDM) to effectively use a transmission path is popularly used. This technique multiplexes light by using a property in which lights having different wavelengths do not interfere with each other. In the WDM optical transmission system, by factors such as an amplification band of an available optical amplifier, and the transmission characteristics of optical sections or an optical fiber, a wavelength band which can be substantially used is limited. A dense wavelength division multiplexing (DWDM) in which inter-signal-light wavelength interval in the WDM is set at 0.8 nm or less (such as 200 GHz and 100 GHz) to multiplex signal lights having 16 different wavelengths is used.
A transmission distance of a signal light is determined by an output in signal light transmission, a loss generated in a transmission path, nonlinear limit, wavelength dispersion, and the like. However, when a repeater having an optical amplifier arranged on an optical path to make it possible to compensate for a signal light level by a loss of the transmission path and to extend the transmission distance. The optical amplifiers includes an optical fiber amplifier which uses an amplification effect obtained by adding a pumping light to an optical fiber added with a rare-earth element and a Raman amplifier which uses a fiber Raman effect obtained by pumping light.
FIG. 20 is a diagram of a configuration which perform Raman amplifier in DWDM optical transmission. As a technique which extends a transmission distance without using a repeater, an optical transmission device in which a pumping light source unit for a Raman amplifier is arranged to insert a pumping light into a transmission path is known (see, for example, Japanese Unexamined Patent Publication No. 2003-134057). In FIG. 20, an optical transmission device 2000 is constituted by an optical multiplexer/demultiplexer 2001 and a plurality of receivers 2002 (2002a to 2002n) corresponding to wavelengths (λ1 to λn) of signal lights to be handled.
Raman amplification is an amplification technique which uses an induced emission phenomenon based on Raman scattering in which amplification is performed in a wavelength band having a wavelength 100 nm longer than the wavelength of a pumping light input to a transmission path 2010. Since the wavelength interval is extremely small, i.e., about 0.8 nm, as shown in FIG. 20, pumping light units the number of which is considerably smaller than the number of signal lights (λ1 to λn) (n wavelengths) are arranged to make it possible to perform Raman amplification to all the signal lights (λ1 to λn).
In a DWDM optical transmission system, signal lights are multiplexed at small wavelength intervals in an available wavelength band, and a signal wavelength is required to be highly stabilized to prevent an adjacent wavelength from being adversely affected. More specifically, expensive optical sections having high wavelength stability, a large-scale apparatus, and technical experts skilled in the art are required. On the other hand, when coarse wavelength division multiplexing (CWDM) in which a wavelength interval of signal lights to be multiplexed is set to be large to generally multiplex signal lights having 8 to 16 wavelengths for one optical fiber is used, a transmission system which includes inexpensive optical sections and can relatively easily make maintenance can be achieved. However, when Raman amplification is performed to extend a reaching distance, a band of a signal light to be amplified becomes wide because a wavelength interval between signal lights of the CWDM, i.e., 10 nm to 60 nm, a band wider than the band covered by the DWDM can be covered. For this reason, a large number of pumping light sources are required. Therefore, a Raman amplifier unit disadvantageously increases in size, and the cost of the pumping light sources disadvantageously increases.